1. Field of the Invention
The invention relates to a pressure operated test tool for use in a subterranean well.
2. Description of the Prior Art
During the course of drilling a subterranean well, the bore hole is filled with a fluid known as "drilling fluid" or "mud". One of the purposes, among others, of this drilling fluid is to contain in the intersected formations any fluid which may be found there. This is done by weighting the mud with various additives so that the hydrostatic pressure of the mud at the formation depth is sufficient to keep the formation fluid from escaping out of formation into the bore hole.
When it is desired to test the production capabilities of the formation, a test string is lowered into the bore hole to the formation depth and the formation fluid is allowed to flow into the string in a controlled testing program. Lower pressure is maintained in the interior of the test string as it is lowered into the bore hole, and this is usually done by keeping a valve in the closed position near the lower end of the test string. When the test depth is reached, a packer is set to seal the bore hole, thus isolating the formation from changes in the hydrostatic pressure of the drilling fluid.
The valve at the lower end of the test string is then opened and the formation fluid, free from the restraining pressure of the drilling fluid, can flow into the interior of the test string.
The test program includes periods of formation flow and periods when the formation is "closed in". Pressure recordings are taken throughout the program for later analysis to determine the production capabilities of the formation. If desired, a sample of the formation fluid may be caught in a suitable sample chamber.
At the end of the test program, a circulation valve in the test string is opened, and formation fluid in the test string is withdrawn.
In an off-shore location, it is desirable to the maximum extent possible, for safety and environmental protection reasons, to keep the blow-out preventers closed during the major portion of the testing procedure and to eliminate test string movement to operate down hole valves. For these reasons, test tools which can be operated by changing the pressure in the well annulus surrounding the testing string have been developed. See, for example, the disclosure of U.S. Pat. Nos. 3,664,415 to Ray, et al, 3,358,649 to Holden, et al, and 3,976,136 to Farley, et al, which patents disclose pressure operated test valves wherein the valve operating force is derived from the action of a trapped inert gas against a piston.
As discussed in the aforementioned patent to Holden, et al, a trapped gas system for operating a test valve necessarily requires the determination of the proper gas operating pressure at the test depth and the insertion of the gas in the tool at the well head at such pressure. In the event of unforeseen changes in the pressure at the formation depth, the test apparatus may readily become inoperative.